You're up against Zuido, the Fleshrender. Initially, you'll also face:

Five Conqueror Army: Zuido Division unions.

Spoiler warning: Boss tactics

The Conqueror Army: Zuido Division unions tend to gang up on one unit at a time.

Zuido tends to use Giant Press every four Turns, which is a leaping, multi-union attack. He will increase its frequency to the end of every Turn once his HP reaches critical.

Zuido sticks to combat arts, except that when his health bar starts to flash, he will cast Miracle on himself. After this, many of his combat arts will take out a union with one blow, so keep a healer on hand.

You'll use a lot of consumables healing your unions in this fight, so make sure you're well-supplied before starting this fight.